Knowledge Bank

The investigation of the pure rotational Raman spectra of gases at high resolution is in progress and is primarily directed to the study of nonpolar molecules. Spectra of allene, allene-$d4$ and dimethylacetylene at gas pressures of 1/2 atmosphere have been photographed in the second order of a 21-ft grating. The spectra have been analysed and the rotational constants have been evaluated. The rotational lines of allene and allene-$d4$ are sharp and well resolved. The lines of dimethylacetylene have a spacing of $0.4cm-1$ and are only just resolved, thus indicating that the method may be used with molecules having B values as small as $0.1cm-1$ and corresponding moments of inertia of about $250\times 10-40$ gm $cm2$. This limitation is probably due to pressure broadening and to the large half-width of the exciting line Hg 4358.